1. Field of the Invention
The present invention relates to the unplanned reaction of energetic materials as a result of exposure to unplanned thermal stimuli. This reaction is termed cookoff. More particularly, the present invention is a process for calculating explosive hazards in munitions by factoring heat flux and chemical venting. Most particularly, the process calculates the heat flux and chemical venting occurring within a given type of munitions which are compared with known critical parameters, allowing a measuring of the possibility of thermally initiated "cook-off" of the munitions.
2. Brief Description of the Related Art
Munitions storage is a hazardous endeavor. One of the primary dangers from storing rockets or missiles arises not just from the explosive charge or warhead, but also the solid rocket propellant itself. In particular, fires and other sources of high ambient heat in proximity to the energetic material create a high risk that the material will prematurely ignite causing a violent energy release.
Cook-off may occur to energetic materials even though they are not directly exposed to an open flame. Ordnance cook-off includes the rapid burning, deflagration, explosion or detonation of a energetic material, such as a propellant, fuze booster charge or explosive in munitions, due to a given amount of heating over a given amount of time. A substantial danger exists that the energetic material will be ignited when it is exposed to high ambient temperatures over a prolonged period of time, generally when the temperature gradually reaches a predetermined ignition temperature, such as temperatures in excess of the critical temperature. Stockpiles of munitions, such as weapons stored in armories, are particularly susceptible to exposure to radiative or convection heat source. Specifically, such an event may occur when a shipboard weapons magazine, which does not contain a fire, is gradually heated due to a fire in a nearby compartment, or other heat source that heats the magazine in which the munition is being stored. The "cook-off" ignition of a single round, in such a case, may trigger the explosion of the warhead, or the ignition and/or explosion of near-by rounds.
Currently, thermally initiated cook-off is predicted from an estimation, based on a constant heat rate, and an Arrenhius rate law for the chemical decomposition of many energetic materials used in the United States Navy. Using this decomposition mechanism to describe the heat release, thermal models of numerous munitions have been constructed dependent on temperature time histories, for predicting munition's cook-off time. Cook-off has caused significant damage on board naval combat vessels. On board vessels, such as aircraft carriers, cook-off has caused increases in damage from deck fires resulting in substantial loss of life and material damage to these capital ships. Fire suppression and fire fighting efforts are inhibited by hazards of ordnance cook-off. With normal operations requiring ordnance to be located on and around aircraft on the carrier deck, a normally controllable fire can degrade the readiness of these ships to perform their mission. Since the occurrence of the catastrophic fires aboard the USS FORRESTAL and USS ENTERPRISE, the U.S. Navy has instituted cook-off improvement programs within the Navy. Significant improvements have been made in increasing ordnance cook-off times, including systems such as heat path interruption techniques, for example, using internal insulating liners and external intumescent coatings. These approaches may slow down the flow of heat into a missile, but if the ordnance is heated for a sufficiently long time, cook-off still occurs. The ability to predict cook-off parameters for the propellant and/or explosive confined within the ordnance readily improves the ability of these ships to increase operational commitments. Ideally, the ability to predict ordnance cook-off includes both slow and rapid rates of heating. Slow cook-off reactions and fast cook-off reactions for the same ordnance item can vary greatly in severity.
There is need for providing cook-off estimations for ordnance under a full range of heat conditions, such as being heated above critical reaction temperatures of the energetic materials inside, and/or with subsequent cooling, exposure to low and/or high heating rates above the critical temperature and other such variations.